Beam systems, such as electron beam systems, ion beam systems, laser beam systems, cluster beam system, and neutral particle beam systems, are used to create features on a surface by etching or deposition. Beam-induced deposition processes use a precursor gas that reacts in the presence of the beam to deposit material on the surface in areas where the beam impacts. For example, a gaseous organometallic compound, such as tungsten hexacarbonyl, is provided near the sample and is adsorbed onto the surface. The organometallic compound decomposes in the presence of a charged particle beam, such as an ion beam or an electron beam, to form a metal that remains on the surface and a volatile organic compound that is removed by a vacuum pump. Etching processes use a precursor gas that reacts with the surface of the work piece to form a volatile compound. For example, iodine can be used to etch a silicon wafer. The iodine reacts in the presence of the beam to form a volatile silicon iodine compound, which leaves the sample surface and is removed by the vacuum pump.
Precursor gases are introduced into the vacuum by a “gas injection system” or “GIS.” Gas injection systems typically include a gas source and a gas director, such as a needle or funnel, that is positioned near the sample and directs the gas toward the work piece. A precursor gas that is generated from a material that is solid or liquid at room temperature is typically supplied from a crucible within the vacuum chamber. The flow of gas is generated by heating the solid or liquid to increase its vapor pressure, causing gas to flow through the gas director and into the vacuum chamber. For example, tungsten hexacarbonyl is a solid at room temperature and is typically heated to about 55° C. or 60° C. to raise its vapor pressure to cause a suitable flow into a vacuum chamber.
One prior art system is described, for example in, U.S. Pat. No. 5,435,850 to Jorgen Rasmussen for a “Gas Injection System.” The gas injection system of Rasmussen includes a crucible in which a solid or liquid source material is stored. The crucible is positioned within the vacuum chamber. The crucible is heated to increase the vapor pressure of the source material, and the gas from the source material then flows to the sample. The gas flow is regulated by the amount of heat supplied to the crucible and by positioning a plunger within a valve to control the size of the valve opening. The limited crucible capacity requires frequent refilling of the crucible in many applications. Such systems require realignment after each refill so that the needle is pointing toward the impact point of the charged particle beam.
Another type of gas injection system is described in U.S. Pat. No. 5,851,413 to Casella for a “Gas Delivery Systems for Particle Beam Processing.” In the systems of Casella, the precursor is stored outside the vacuum chamber, and flows through a conduit into a gas concentrator near the sample. Systems that store the precursor gas outside the vacuum chamber typically include a valve, such as a stepper-motor-controlled diaphragm valve, to control the gas flow.
US Pat. Pub. No. 2009/0223451 describes a system for delivering precursor gases to a beam instrument. The system uses a carrier gas to dilute and carry the precursor gases from one or more crucibles though a single main line to a needle and into the sample vacuum chamber. Flow of the carrier gas and the gas from each crucible is controlled in part by controlling the duty cycle of a pneumatic valve. Part of each crucible and the main line are in a gas envelope that opens to the sample vacuum chamber. Use of a single main line leaves precursor gas in the main line when the crucible valve is closed, thereby requiring a purging procedure for the main line, which takes time and wastes precursor gas.
U.S. Pat. Pub. No. 2011/0114665 A1 by Chandler et al. for a “Gas Delivery for Beam Processing Systems” addresses the issue of managing the sample chamber pressure, which was a limitation of the proceeding inventions. In Chandler's delivery system the gas flow from multiple gas sources is controlled by a cycling valve controlling the flow from each gas source, with the gas pressure in the sample chamber being determined by the relative time that the valve is opened and the upstream pressure at the valve. A gas valve positioned inside the vacuum chamber allows rapid response in shutting off a gas. This method of gas flow regulation is known as pulse width modulation (PWM).
All these system take time to establish the correct flow through the needle and into the sample chamber. While the gas flow is being adjusted, the sample and other component in the vacuum chamber are being exposed to an incorrect flow of incoming gas. Moreover, because gas molecules tend to stick to the surfaces inside the gas injection system, it takes some time after a new gas is introduced before the previously used gas is no longer present in the flow.
As the demands of gas-assisted beam processing increase and processes are required to produce ever finer structures, applicants have found that this lack of control can adversely affect the processing results. Controlling the gas flow is particularly important in sensitive processes that use multiple gases, such as the process described in U.S. Pat. Pub. No. 2010/0197142 by Randolph et al for “High Selectivity, Low Damage Electron-Beam Delineation Etch.”